(1) Field of the Invention
This invention relates to rim type attenuating phase shifting masks used for forming contact holes and more specifically to the use of different transmittance phase shifting materials for pattern regions having different duty ratios.
(2) Description of the Related Art
Attenuating phase shifting masks find wide usage in the printing of contact holes on a layer of photoresist formed on an integrated circuit wafer. In those patterns which have contact holes on dense sub patterns and contact holes on more isolated sub patterns it is a problem to optimize the mask for the different sub pattern regions. At lower sigma settings the depth of focus is greater for the isolated pattern than for the more dense pattern, however side lobe effect problems become more significant. At higher sigma settings the depth of focus is smaller for the isolated pattern than for the more dense pattern, however side lobe effect problems are decreased. The value of sigma is a measure of the degree of coherence of the light used to project the mask pattern. A smaller the value of sigma indicates a greater coherence of light. For completely coherent light the value of sigma is zero. For incoherent light the value of sigma is 1.0. The value of sigma is variable in the exposure system and depends on the process settings. Projection systems typically use light having a value of sigma of about 0.6.
The depth of focus can be increased by increasing the transmittance of the attenuating phase shifting material but this causes side lobe problems in the dense portions of the mask. Trade-offs are necessary between the depth of focus of the different pattern regions; problems due to side lobe effect; and the degree of coherence of the light used to project the mask pattern.
U.S. Pat. No. 5,725,973 to Han et al. describes a photomask for suppressing proximity effect in a mask pattern. The proximity effect is suppressed by forming an optical transmittance control film pattern in the transmission area between opaque mask patterns.
U.S. Pat. No. 5,786,114 to Hashimoto describes a phase shift mask having phase shift portions of both Levenson type and Halftone type. A method of controlled oxidation of the halftone material to increase and stabilize the transmittance of the halftone material is described.
U.S. Pat. No. 5,882,827 to Nakao describes a phase shift mask of both Levenson type and Halftone type.
U.S. Pat. No. 5,849,439 to Mitsui describes a method of controlling the transmittance and thickness of phase shifting material using a reverse sputtering process.
U.S. patent application Ser. No. 09/473,027 (TSMC-99-118), Filed Dec. 28, 1999, entitled "USING DIFFERENT TRANSMITTANCE WITH ATTENUATE PHASE SHIFT MASK (APSM) TO COMPENSATE ADI CRITICAL DIMENSION PROXIMITY", and assigned to the same Assignee describes the use of different transmittance light absorbing materials for forming line/space patterns and contact hole patterns having different duty ratios.